I'd be doing the exact same thing if I was Forest so it's hard to get mad at them. Are they taking the piss? Yes, they're taking the piss and why wouldn't you.
What exactly is making Dean Henderson an appealing proposition right now for PL outfits? He was one of the lower performing keepers on some metrics. They're being cheeky because they can afford to be cheeky after some average at best PL performances. It would be different if we were garnering interest from 2 or 3 PL clubs which would sharpen Forest's minds but that doesn't seem to be the case. It's probably hard to drum up interest in a keeper on 120k a week that put in those performances that were okay at best. The only reason we have Forest at the end of the line is they had him and he was okay so they're probably thinking better going for a keeper we know, but only if it's a very good deal.
The big mistake we made with Dean Henderson is not making a decison earlier in his career and putting him on far, far too much money for what he'd shown at that point in time. We made a rod for our own backs, a recurring theme with some of these sales that really comes back to bite us on the ass.